What's the difference between these 2 description?

Hi, well as you remember Amit mentioned that everything in Objective-C is a pointer, so what is happening when you allocate and initialize a variable:

NSNumber *mike = [[NSNumber alloc] initWithInt:23];

Notice that mike is only a pointer to a particular cell in memory, then you use the class method on NSNumber alloc, so alloc will allocate memory for some data (in this case it is NSNumber data type) and return it, so:

[NSNumber alloc] will return a memory address something like 0x00ab1628ff and then notice that we call another method on the memory that has been allocated for us:

[[...] initWithInt:23] - here you initialize the variable using the memory that has been allocated for it and putting some data that is of type NSNumber and was made with an integer into that memory cell.

But the funniest part is that you can use:

NSNumber *mike = @23 - here @ which is specific to Objective-C only does all the heavy lifting for your manual labor, allocates memory and initializes the variable (putting the data into the memory cell) automatically for you, hope it helps